FIGS. 50 to 52 show an outer panel for a side structure of a conventional railroad car. An outer panel 101 (side structure) is provided with an window opening 102, and is provided with door pocket parts 107 and 108 and door openings 103 and 104 arranged at fixed intervals on the opposite sides of the window opening 102, respectively. The door pocket parts 107 and 108 are provided with reinforcing members 105A and 105B provided with vertical members 105a having a substantially U-shaped cross section, and fixed in place by welding. Second reinforcing members 105C and 105D provided with vertical members 105b having a substantially U-shaped cross section are disposed in upper parts (parts at a height corresponding to the height of the window opening 102) of a part between the first reinforcing member 105A and the window opening 102 and a part between the first reinforcing member 105B and the window opening 102, respectively.
Three parallel third reinforcing members 105E to 105G having a substantially hat-shaped cross section are extended longitudinally in a lower part, extending under the window opening 102, of a part extending between the first reinforcing members 105A and 105B. Two vertical fourth reinforcing members 106A and 106B having a substantially Z-shaped cross section are extended perpendicularly to the third reinforcing members 105E through 105G in the lower part extending under the window opening 102. Side posts 106C and 106D, i.e., rim members, having a substantially Z-shaped cross section and a height greater than that of the fourth reinforcing members 106A and 106B are set on the opposite sides of the window opening 102, respectively. A fifth reinforcing member 106E having a substantially Z-shaped cross section is extended longitudinally under the window opening 102 and above the third reinforcing members 105E to 105G. Vertical door posts 106F and 106G having a substantially Z-shaped cross section are set vertically on the opposite sides of each of the door openings 103 and 104.
The side posts 106C and 106D are welded to a part between the one of the opposite sides of the window opening 102 and the door pocket part 107 and a part between the other side of the window opening 102 and the door pocket part 108, respectively, of the outer panel 101, and the door posts 106F and 106G are welded to a part between the door pocket part 107 and the door opening 103 and a part between the door pocket 108 and the door opening 104, respectively, of the outer panel 101.
Those frame members including the side posts and the door posts maintain the shape of the side structure, and the plurality of reinforcing members maintain the shapes of parts, between the frame members, of the outer panel.
Those reinforcing members 105A to 105G, 106A, 106B and 106E are welded to the outer panel 101 to secure the buckling strength of the outer panel 101 of the body structure of the car. Various body structures have been proposed.                (1) A side structure of an electric passenger car proposed in JP61-220962A is constructed by welding corrugated reinforcing plates to the inner surface of a side outer plate, and filling up spaces between the reinforcing plate and the side outer plate with a core material.        (2) A side structure proposed in Japanese Patent No. 3015020 is formed by processing a stainless steel double-skin panel having outer and inner panels each serving as a load-bearing member. The outer and the inner panel have substantially the same thickness and are formed of the same material. This side structure is provided with core members formed by bending a stainless steel plate.        
To fabricate the side structure mentioned in JP61-220962A, reinforcing plates of different sizes respectively corresponding to those of regions to be reinforced of the side outer panel need to be made, which requires troublesome processes because the reinforcing plates are corrugated. Since welding amount necessary for welding the reinforcing plates to the outer panel of a side structure is the same as that necessary for constructing a conventional frame-skin structure, the prior art body structure has no effect on reducing welding distortion. In filling up the spaces with a filling material (core material), the reinforcing plates need to have a thickness sufficient to withstand pressure due to the foaming of the filling material. Consequently, the weight of the reinforcing plates and the filling material is greater than that of a conventional buckling-preventing reinforced structure.
The technique mentioned in Japanese Patent No. 3015020 uses both the outer and the inner panel for bearing load. Therefore, the inner panel has a thickness substantially equal to that of the outer panel, is a high-strength member formed of the same material as the outer panel, and hence is considerably heavy. In constructing the side structure, it is necessary to bond together not only the outer panels but also the inner panels to bear load by the inner panels as well as the outer panels, which needs complicated welding construction. Spaces between the outer and the inner panel cannot effectively be used for thermal insulation, wiring and piping.